Trace elements analysis in steel and vacuum cast superalloys

Vacuum/volume 43/numbers Printed in Great Britain

5-7lpages

717 to 721 I1992

0042-207x/92$5.00+.00 @ 1992 Pergamon Press Ltd

Trace elements analysis in steel and vacuum cast superalloys AOsojnik and T Drglin, Institute of Metals and Technologies, Lepipot 11, 67000 Ljubljana, Slovenia A new sensitive analytical method for low level As and Bi determination in non-oriented electrical steel sheets and vacuum cast nickel superalloys was developed at our institute. Hydride generation/atomic absorption spectrometry (HG AAS) has proved to be a suitable method for the determination of hydride-forming elements, particularly at a low concentration level. The chemical and instrumental parameters were optimized. The chemical interferences of matrix elements and some metalloids were studied and eliminated. Certified standard reference materials were used to verify the method. The sensitivity, accuracy and reproducibility of data, and also the practical detection limit for each element, are gathered and assessed.

1. Introduction Mechanical, physical and technological properties of various types of steels, and especially vacuum cast superalloys, for high temperature application strongly depend on trace element contents such as Bi, As, Sb, Sn, Te, Se, Pb, Ag, and others. Because of their harmful effect existing at the pg gg ’ levels and lower, great attention was made to reduce the concentration of these elements to a minimum (vacuum casting). This is especially true for bismuth in high temperature resistant nickel alloys as the allowed concentration is strongly limited (0.1-0.5 pg g- ‘. depending upon the alloy type and application purpose). Therefore, the determination of these elements is extremely important and the development of a suitable, sensitive analytical method is necessary. Hydride generation AAS provides a sensitive and selective method for the determination of the hydride-forming elements, particularly at a low concentration level. The advantage of this method lies clearly in the separation and enrichment of the analyte element by the volatization of gaseous hydrides. The only disadvantage of the method is the narrow range of linearity, because atomic absorption units are not very linear in response. The hydride-forming elements may often be present in several oxidation states, which can produce significantly different sensitivities. Several authors’-’ have observed that the higher oxidation state indicates generally lower sensivitity. This can probably be explained by the different rates of hydride generation. It is therefore desirable that only one oxidation state be present before reduction by sodium borohydride occurs. Interferences caused by metal ions are a common problem in the generation of hydrides. In the presence of metals such as nickel, cobalt, copper and others, the absorption signal for the hydride-forming element is severely suppressed6 ‘. This can be interpreted as a preferential reduction of the interfering ion to the elemental state, resulting in co-precipitation of the analyte or adsorption of the volatile covalent hydride formed’ “. The interference caused by these elements can be partly or completely eliminated. The most common way to eliminate the interferences is masking of interfering ions by different masking agent 7.I 3.14,The use of iron (III) to eliminate Ni and Cu interferences was studied by several authors”-“. The mechanism

involved is probably due to a preferential reduction of Fe3+ to Fe’+ before the reduction of the interfering elements occurs. The present work involved an extensive study about : (i) hydride generation conditions ; (ii) influence of oxidation state ; m) interferences of matrix elements an:“ (iv) evaluation of results.

and some metalloids

;

2. Experimental 2.1. Apparatus. A Perkin-Elmer 2380 AA spectrometer, equipped with hydride generator MHS-10 was used. The hydride generation system and mechanism of hydride formation are shown in Figure 1. The instrumental parameters and operating conditions are listed in Table 1. 2.2. Reagents and solutions. All reagents were of the highest available purity. The solutions prepared were 3% sodium tetrahydroborate stabilized with 1% sodium hydroxide, 10% EDTA, 10% thiourea, 0.01 M 1.10 phenanthroline and 10 mg ml- ’ Fe’+ solution, prepared by dissolving high purity iron in aqua regia. 2.3. Standard solutions. A stock solution of 1000 pg ml- ’ As3+ was prepared by dissolving the appropriate amount of As,O, in 20 ml of 1 mol I-’ NaOH neutralizing the solution with 1 mol I- ’ HCl and diluting to one litre with 1.5% HCl. Stock solution of 1000 pg mll ’ As’+ was prepared by oxidation of an appropriate amount of As3+ standard solution with hot nitric acid and diluting with 1.5% HCI. A stock solution of 1000 pg ml-’ Bi3+ was prepared by dissolving 1 g of bismuth metal with 20 ml of aqua regia and diluting to one litre with 1.5% HCI. The other standards solutions were prepared from stock solutions by diluting with 1.5% HCl. AS’+ and Bi3+ solutions containing the interfering ions were prepared by adding the appropriate amounts of chloride salts to the test solutions. 2.4. Sample preparation. The sample 0.1-l solved in 20 ml of aqua regia and diluted

g, was carefuly disto 100 ml. A 0.5-5 717

A Osojnik and T&glint

Trace elements analysis 100

RUARTZ

in 10 ml 1.5 ‘I0 HCI

ng Ass+

CELL

123156 0,100 0.300 0,200

REACTION

0,100 IL123456

MECHANISM

OF

M3++ 3BH;

M = Metal

Figure I. Hydride spectrometer.

HYDRIDE

FORMATION

+ $3 + 2H+ -----) MH3+H2+2 AT MHj-M (Bi,As,Sb,Se,Te

generation

system

MHS-IO.

B2H,j

) Perkin-Elmer

IIl_.._i-_L

2380 AA

123456 ADDITION

ml aliquot of this solution was injected into the reagent flask of the hydride generator, which contained 10 ml of I .5% HCI. The addition of 5 ml IO’?? EDTA (for arsenic determination) or 1 ml of IO mg ml- ’ Fe3+ (for bismuth determination) was to eliminate the interferences. The argon was led through the solution and a reductant (NaBH,) was introduced into the reaction flask. The absorbance was measured and evaluated from peak heights.

Figure 2. Influence

OF INTERFERING

of the matrix elements

concentrations

much

higher

than

in samples

investigated.

Imgl

’

on the signal for As’

50 ng Bi3’ in 10 ml 1.5 “/, HCL

2.5. Calibration. Calibration was carried out with standard solutions in the range from 10 to 100 ng As’+ and Bi3+, respectively. The method of standard addition was also applied. 3. Results and discussion. The interfering efTect of the matrix elements on the signal of As’+ and Bi’+ are shown in Figures 2 and 3. As indicated the presence of Ni, Co and Cu depress the signal for AsS+ and Bi”+ strongly. The interferences of hydrideforming elements Sb, Sn, Se, Te in the arsenic determination and Se, Te in the bismuth determination were stated only in

ELEMENT

0

1

2

3

4

5

6

MO

LA

0 0,400

E

0.

1

2

3

L

5

6

I

r

.

r

?

Fe 0

0,2 0

0.100

Table 1. Instrumental Spectrometer Wavelength Slit Light source Hydride system Calibration volume Diluent Reductant Flame

718

Parameters

and operating

Perkin-Elmer 2380 193.7 nm (As) 223.0 nm (Bi) 0.2 nm (As) 0.7 nm (Bi) EDL As-9.5 W EDL Bi--8.0 MHS-IO 10ml 1.5% HCI 3% NaBH, in 1% NaOH Air/acetylene : lean, blue

I

01

0123456 0,400

conditions

Ti

0.3

l-----A

QMO W

23156

0,100 @I!zzl 0 1

2

ADDITION

Figure 3. Influence

3

4 OF

5

6

INTERFERING

of the matrix elements

0

I

1

I

1

2

3

ELEMENT

I

6

I

I

5

6

Cmgl

on the signal for Bi’

’

Trace elements

A Osojnik and T Drglin:

analysis

5Ong Bi3’ In 1Oml 1,5%HCI+Ni(3mg)Co(lmg),~J(0.5mg),

100 ngAs5’+ Ni (3mg), Co (lmg), MO(lmg). Cu (OSmg), Sb(O,lmg), Sn(O,Olmg),Se(Qlmg), Te(O;lmg)+ EDTA

m

O.LoO

0,300

ngAs

Se(O.lmg). 0.300

l

0,300

5011 Bi

Ni

0,200

Ni

0.20

2

100

0,600

3

_.1_L 4

A+’

0,300

MO

~

0

0.200

2

1

6

0

1

2

3

4

1

5

2

3

1

co

2

L

6

8

10

2

L

6

8

10

0

10

0,100 r-

Bi

0,100

0.100

5

50”

0.200

0,100

1

m

Te(0,1mg)+k3’

5

100 ngAS

E 1

2

3

Sn

I

Q

L

0

5

2

4

6

8

10

U.-r””

100ng As

l

a300

Se

0,200

c

t-----

0,100

%==wF=d ml 10 “/. EDTA

Figure 4. Elimination

of interferences

1

4 5 2 3 ml 10 % EDTA

with the addition

0

of EDTA-for

ASS+.

mg Fe3’

has been proved by 1.10 phenanthroline, thiourea, EDTA and Fe3+. The use of 1.10 phenanthroline and thiourea as masking agents did not improve the signal for arsenic and bismuth in the presence of interfering elements because of foaming of the solution by the addition of the masking agent. The interferences of matrix and some hydrideforming elements were eliminated by EDTA for arsenic determination (Figure 4) and by the addition of Fe3+ for bismuth determination (Figure 5). The interfering effect of Sb and Se on the AS’+ signal and of Se on the Bi3+ signal could not be eliminated in this way The influence of oxidation state on the sensitivity of arsenic is shown in Figure 6. The signals for As3+ are about 25% higher than for As5+. Regarding the sample decomposition (aqua regia) and time consuming reduction of As5+ to As3+ with potassium iodide the calibration with As5+ standard solutions was chosen in spite of higher sensitivity for AsI+. The calibration curve made with pure As’+ standard solutions solutions considering the matrix and with As’+ standard elements is given in Figure 6. The same curve was obtained for nickel alloys (BCS 345) and for steel samples (BCS 365) which enables the use of the same curve for different types of samples. The difference between the both curves indicated in Figure 6, lies probably in the different volumes of solutions used by the experiment. Owing to this, the evaluation of results has to be performed by the calibration curve made by the standard addition method. Standard reference materials of steel and Ni alloys containing 0.2-21 pg gg ’ of Bi and 0.5-100 pg gP ’ of As were used to test the method. The RSD in this range is k 16% to f 6% for Bi and Elimination

of interferences

Figure 5. Elimination

of interferences

J

mg Fe3’ with the addition

of Fe3+-for

Bi’+.

+20% to *3% for As. The detection limit is 0.1 pg gg ’ for Bi and 0.5 pg g-’ for As. The linearity of the calibration curve is from 10 to 100 ng for both elements. The results of As and Bi determination in steel and Ni alloys samples are given in Tables 2 and 3, respectively. The results indicated good agreement within the reported standard deviations for both elements. 4. Conclusions The interferences of Ni, Co, Cu due to reduction of interfering elements to elemental state and kinetic changes of hydrideforming reaction were established. The interferences of hydride-forming elements due to disturbances in the gas phase were stated only in concentrations much higher than in samples investigated. For arsenic determination addition of EDTA was selected to eliminate the interferences. For bismuth determination the best reagent for interferences elimination was Fe3+. This action is probably due to the preferential reduction of Fe3+ to Fe2+ before the reduction of the interfering element occurs. The evaluation of results has to be performed by the calibration curve made by the standard addition method. Certified standard reference materials of steels and Ni alloys were used to verify the method. RSD in this range is f20 to f 3%. The detection limit is 0.1 pg g- ’ for Bi and 0.5 pg g- ’ for As. The linearity of the calibration curve is 10-100 ng for both elements. 719

A Osojnik and T Drglin: Trace elements analysis

As5+

As3+ ABS.

ABS. . As3’

standard

solutions

o As 3’ standard

0,600-

solutions+

EDTA,,

0,600

+ EDTA + BCS 345 0,500

. As5’standard oAs5’ standard xAsS+ standard

solutions solutions+EDTA solutions+EDTA+BCS345

+Ass+ standard

solutions+

EDTA +BCS365

(0,5g/lOOm~-0,5ml) i

0,200 I

I 25 Figure 6. Calibration

I

I

50

75

I

LF IL_-

100 ng As3+

25

50

75

100ngAs5+

curves

Table 2. Results of As determination

Table 3. Results of Bi determination Certified value (%g-‘)

Analysis results

BCS 345 IN 100

<0.2

<0.2

4.2

BCS 346 IN 100

10

9.8

0.4

4.1

2

8.7

MBH 11982A IN 100

2

1.9

0.3

14.2

1.2

0.2

16.7

MBH 11982B IN 100

0.7

0.74

0.1

16.2

1.7

0.2

11.8

MBH 11980F IN 100

<0.5

<0.2

2

2.4

0.2

8.3

No 0X574/2 Nimonic 263

NBS 363 Cr-V steel

100

96

3

3.1

I.2

6.0

BCS 464 Aust. stainless steel

30

29

1

3.4

MBH BS 12 Chill cast low alloy steel

5.6

No 1 Non-oriented steel sheet

(/%g-‘)

Analysis results (!%g-‘)

*s (/% g- ‘)

+ RSD (%)

CRM sample No

BCS 345 IN 100

1.4-3.0

1.0

0.2

20.0

BCS 346 IN 100

50

48

2

MBH 11982B IN 100

25

23

MBH 11980F IN 100

<5

Certified value

CRM sample No

No 0857412 Nimonic 263 NBS 365 Electrolytic

iron

No 1 Non-oriented steel sheet

The method and excellent

18

offers sensitivity

satisfactory for both

accuracy,

I

good

reproducibility,

elements.

(/%g-‘)

<0.2 21

20

<0.2

‘J R Castillo, J M Mir, C Martinez and M T Gomez, Fr Z Analyt Chem, 325, 171 (1986). 4K Petrick and V Krivan, Fr Z Anal Chem, 327,338 (1987). 5 K De Doncker, R Dumarey, R Dams and J Hosle, Analyt Chim Acta,

153,33 (1983). References

’ R Bye, Talanta, 37, 1029 (1990). ‘T Guo, W Erler, H Schulze and S McIntosh, (1990). 720

Atom Spectrosc, 11, 24

‘K De Doncker, R Dumarey, R Dams and J Hoste, Analyt Chim Acta. 169, 339 (1985). ‘Y Yamamoto and T Kumamura, 2 Ana/yt Chem, 281, 353 (1976). RJ R Castillo, J Lanaja, M C Martinez and J Aznarez. Analysf, 107, 1488 (1982).

A Osojnik and T Drglin : Trace elements

analysis

‘J Sanz, M T Martinez and J Galban, J Analyt Atom Spectrom, 5, 651 (1990). “‘M Verlinde and H Deelstra, Fr 2 Analyt Chem. 296,253 (1979). “G F Kirkbright and M Taddia, Analyt Chim Acta, 100, 145 (1978). 12A E Smith, Analyst, 100, 300 (1975).

I3J E Drinkwater, Analyst, 101, 672 (1976). I4 R Bye, Fr Z Analyt Chem, 317,27 (1984). “T Wickstrom and W Lund, Analyt Chim Acta, 208, 347 (1988) ’ 6B Welz and M Melcher, Analyst, 109, 577 (1984). “R Bye, Analyf Chim Acta, 192, 115 (1987).

721